Method and device for producing gaseous compressed oxygen having variable power consumption

ABSTRACT

Variable production of compressed oxygen by means of low-temperature separation of air in a distillation column system having a high-pressure column and a low-pressure column. In a first operating mode, a first total air quantity is cooled in the main heat exchanger, and a first turbine amount is fed to the expansion to perform work. In a second operating mode, a second oxygen stream from an external source outside the distillation column system is introduced into the low-pressure column in a liquid state. There is less total air cooled in the main heat exchanger, and less air is fed to the expansion to perform work than in the first, operating mode.

The invention relates to a method for variable production of gaseous compressed oxygen having variable energy consumption according to the preamble of claim 1.

Methods and devices for low temperature separation of air are known, for example, from. Hausen/Linde. Tieftemperaturtechnik [Low temperature technology], 2nd edition 1985, chapter 4 (pages 281 to 337).

The distillation column system can be constructed as a two-column system (for example as a classic Linde double-column system), or else as a three-column or multicolumn system. In addition to the columns for nitrogen-oxygen separation, the system can comprise further devices for producing high-purity products and/or other air components, in particular noble gases, for example an argon production and/or a krypton-xenon production,

In the process, a liquid pressurized liquid product stream is vaporized against a heat carrier and finally produced as gaseous compressed product. This method is also termed internal compression. It serves for producing compressed oxygen. For the case of a supercritical pressure, no phase transition in the actual meaning takes place, and the product stream is then “pseudo-vaporized”.

A heat carrier at high pressure is liquefied (or pseudo-liquefied, if it is below a supercritical pressure) against the (pseudo)-vaporizing product stream. The heat carrier is frequently formed by a part of the air, in the present case the “second substream” of the compressed feed air; sometimes this stream is also termed throttle stream, although, instead of a throttle valve, expansion can alternatively proceed in a liquid turbine (DFE—“dense fluid expander”).

Internal compression methods are known, for example, from DE 830805, DE 901542 (=U.S. Pat. No. 271,238/U.S. Pat. No. 2,784,572), DE 952908, DE 1103363 (=U.S. Pat. No. 3,083,544), DE 1112997 (=U.S. Pat. No. 3,214,925), DE 1124529, DE 1117616 (=U.S. Pat. No. 3,280,574), DE 1226616 (=U.S. Pat. No. 3,216,206), DE 1229561 (=U.S. Pat. No. 3,222,878), DE 13 99293, DE 1187248 (=U.S. Pat. No. 3,371,496), DE 1235347, DE 1258882 (=U.S. Pat. No. 3,426,543), DE 1263037 (=U.S. Pat. No. 3,401,531), DE 1501722 (=U.S. Pat. No. 3,416,323), DE 1501723 (=U.S. Pat. No. 3,500,651), DE 253132 (=U.S. Pat. No. 4,279,631), DE 2646690, EP 93448 B1 (=U.S. Pat. No. 4,555,256), EP 384483 B1 (=U.S. Pat. No. 5,036,672), EP 505812 B1 ( U.S. Pat. No. 5,263,328), EP 716280 B1 (=U.S. Pat. No. 5,644,934), EP 842385 B1 (=U.S. Pat. No. 5,953,937), EP 758733 B1 (=U.S. Pat. No. 5,845,517), EP 895045 B1 (=U.S. Pat. No. 6,038,885), DE 19803437 A1, EP 949471 B1 (=U.S. Pat. No. 6,185,960 B1), EP 955509 A1 (=U.S. Pat. No. 6,196,022 B1), EP 1031804 A1 (=U.S. Pat. No. 6,314,755), DE 19909744 A1, EP 1067345 A1 (=U.S. Pat. No. 6,336,345), EP 1074805 A1 (=U.S. Pat. No. 6,332,337), DE 19954593 A1, EP 1 134525 A1 (=U.S. Pat. No. 6,477,860), DE 10013073 A1, EP 1 339046 A1, EP 1146301 A1, EP 1150082 A1, EP 1213552 A1, DE 101 15258 A1, EP 1284404 A1 (=US 2003051504 A1), EP 1308680 A1 (=U.S. Pat. No. 6,612,129 B2), DE 10213212 A1, DE 10213211 A1, EP 1357342 A3 or DE 10238282 A1, DE 10302389 A1, DE 10334559 A1, DE 10334560 A1, DE 10332863 A1, EP 1544559 A1, EP 1585926 A1, DE 102005029274 A1, EP 1666824 A1, EP 1672301 A1, DE 102005028012 A1, WO 2007033838 A1, WO 2007104449 A1, EP 1845324 A1, DE 302006032731 A1, EP 1892490 A1, DE 102007014643 A1, A1, EP 2015012 A2. EP 2015033 A2, EP 2026024 A1, WO 2009095188 A2 or DE 102008016355 A1.

Frequently, a fluctuating oxygen demand forces an air separation plant to be designed for variable operation with variable oxygen production. Conversely, it can be logical to operate an air separation plant variably, despite constant or substantially constant production, by providing differing modes of operation which have different levels of energy consumption.

Owing to differing factors (not least a constantly increasing fraction of renewable energies in power generation), the power tariff variations in the industrial plant sector are continually becoming greater. Affected by certain seasonal variations, the breadth of the variation of the power tariff is also determined by the day-night cycle.

For a low power demand in the grid (for example during the night), there can be a surplus of power. This surplus, however, needs to be withdrawn, and is therefore offered at a lower price. If the power demand in the grid (for example during the day) increases, the power price also increases. Depending on the region and special framework conditions, the power prices at a location can vary by a factor of five or even greater.

There is therefore a need to equip air separation plants with rapid and efficient adaptation to load. Shutting off such a plant for a short time is generally impossible, owing to a delivery of gaseous compressed oxygen that needs to be maintained continuously.

It has already been known for over 30 years to use exchangeable storage methods in order to compensate for a varying energy supply (Springmann, “Energieeinsparung” [Energy saving], Linde-Symposium “Luftzerlegungsaniagen” [Air separation plants], 4^(th) Conference of Linde AG of Oct. 15-17, 1980, article H). However, these require a relatively high expenditure in terms of apparatus and control technology. In addition, U.S. Pat. No. 7,272,954 discloses, in the case of a high power price, the introduction of low-temperature liquid into the distillation column system and the use of the excess refrigeration by means of a cold compressor; here also, however, additional expenditure in terms of apparatus is necessary.

A method as per the preamble of patent claim 1 is known from EP 793070 A2.

The object of the invention is to specify a method of the type described at the outset and a corresponding device which require a comparatively low expenditure on apparatus, nevertheless permitting operation of the plant that Is variable in a particularly wide range with respect to the energy consumption thereof, and operate particularly efficiently here.

This object is achieved by the features of the characterizing part of patent claim 1.

With a low energy supply and a high electric power price, the plant is run in the second operating mode. In this case, by feeding in liquid oxygen, not only is cold introduced into the plant, but also separation work that is already done. The oxygen which is fed from outside no longer needs to be produced in the plant. Correspondingly, the total air amount which is introduced into the plant can be reduced. The production of cold can also be reduced, in an extreme case to zero. The turbine stream (second substream) is therefore reduced or even shut off. In this case the amount of gaseous compressed oxygen product remains the same or substantially the same. “Substantially the same” is here taken to mean a change by less than 3%, preferably less than 2%.

In the invention, two parallel-connected booster compressors (also termed “booster air compressors” (BACs)) are used for the second and third substreams of the air; in other words, the corresponding booster compressor is of a double-stranded design. This gives rise to a particularly high bandwidth in which the total amount of feed air and therefore the energy consumption of the plant can be varied. Compared with a first mode of operation which is constructed as a design case with high liquid production, the energy consumption in a second mode of operation is reduced to 50%, by turning off one of the two booster compressors and operating the other in underload (for instance 0%). The main air compressor in which the total air stream is first compressed can in this case likewise be of a muitistranded or optionally single-stranded design. The two booster compressors have, for example, 2 to 5 stages, in particular 3 to 4 stages. Of course, in the invention, three or more parallel-connected booster compressors cars also be used for the second and third substreams of the air; the booster compressor is then designed to be of three-stranded or muitistranded construction. Upstream or downstream of the muitistranded booster compressor, further booster compressors can be used which compress the second and third substreams individually or together.

In the context of the invention, the first pressure (first substream, termed throttle stream) and the second high pressure (second substream, termed turbine stream) can be identical or different. The total air can also be compressed to the first or second high pressure; alternatively, the total air can be compressed to a lower pressure, tor example to the high-pressure column pressure plus line losses, and the first and/or the second substream of the air booster-compressed. The second substream, after the work-producing expansion thereof, is generally introduced into the high-pressure column at least in part, preferably completely, or substantially completely.

The “total air stream” is taken to mean the amount of air which is introduced into the distillation column system in the end effect. This takes place in different ways, in the form of two, three or more substreams which flow through the main heat exchanger on at least one subpiece.

The liquid oxygen to be fed in (second oxygen stream) in the second mode of operation can be produced in the plant itself during the first mode of operation (“third oxygen stream” of patent claim 3); the “external source outside the distillation column system” is then formed by a liquid oxygen tank into which, during usual operation, at least a part of the third oxygen stream is introduced. Alternatively, the second oxygen stream can be withdrawn completely, in part or at times from another source, for example from a liquid tank which is not from the distillation column system of the plant, but is filled from that of an adjacent air separation plant or from tanker trucks.

In standard operation of the plant, in the distillation column system, in addition to the liquid oxygen, further liquid products such as liquid nitrogen and/or liquid argon can be produced.

It is expedient when in the invention at least one, preferably all of the conditions stated in patent claim 2 are met. Preferably, the streams in the second mode of operation (operation during reduced energy supply) are reduced relative to the first mode of operation (standard operation with liquid production) to a value which is in the following numerical ranges:

Total air amount 5 mol % to 30 mol % Turbine amount (turbine stream) 10 mol % to 100 mol %

Regularly, in the second mode of operation, no liquid product is produced, or, if an argon production is provided, no liquid product apart from argon.

A particularly effective adaptation to a fluctuating energy supply may be achieved by a method as claimed in patent claim 3, in which, in the first mode of operation (in the standard mode) a third oxygen stream is taken off from the low-pressure column as liquid product. In the second mode of operation (electric power-saving operation), less oxygen is obtained as liquid product, preferably none at all. The second liquid oxygen amount (of LOX product) is preferably lower by 50 mol % to 100 mol % than the first liquid oxygen amount.

In the second mode of operation, preferably none of the process streams of the distillation column system is subjected to a cold compression. In particular, in the second mode of operation, no rotating machines are used that are not also used in the first mode of operation. The expenditure on hardware for the variable operation is therefore extremely low.

“Cold compression” here is taken to mean a gas compression operation in which the gas is fed to the compression at a temperature which is markedly below the ambient temperature, in particular below 240 K.

The method according to the invention can be carried out particularly efficiently thereby. The entire cold which is fed via the liquid feed can be utilized in order to reduce the turbine air amount. In that correspondingly less air must be booster compressed, or in that—in the case of methods having compression of the total air to a high pressure—the total air is compressed to a markedly lower pressure.

Preferably, in the second mode of operation, the work-producing expansion of the second substream is entirely shut off, that is to say the second turbine amount is zero.

The two booster compressors can each have a separate aftercooler; alternatively, the heat of compression thereof is removed in a shared aftercooler.

In principle, the total air stream can consist only of the first substream (turbine stream) and the second substream (throttle stream). The total air stream can also comprise further air substreams, including a first part (direct air) which is fed without turbine expansion and in substantially the gaseous state into the distillation column system, in particular into the high-pressure column. As “substantially gaseous”, here, a stream is designated which is completely gaseous or contains less than 1-2 mol % liquid. Preferably, the total air stream is divided into exactly three air streams, as is described in patent claim 7.

The invention further relates to a device according to patent claim 8. The device according to the invention can be supplemented by device features which correspond to the features of the dependent method claims.

The variable mode of operation according to the invention can be applied not only to plants which from the start are designed for such a variable operation; rather, the invention also relates to a method for retrofitting an existing low-temperature air separation plant according to patent claims 9 to 11.

In this ease, there is virtually no need to intervene in the hardware of the existing distillation column system. If a line for feeding in liquid oxygen into the low-pressure column is absent, it naturally needs to be retrofitted. In some circumstances, an existing line can also be utilized; then, only fittings and optionally a pump need to be supplemented. Moreover, the controller is adapted, that is to say the software of the operating control system. In particular, no rotating machines need to be retrofitted. One exception can be the second booster compressor if the existing plant has only a single-stranded booster compressor.

The invention and also further details of the invention will be described in more detail hereinafter with reference to exemplary embodiments shown schematically in the drawings, In this case:

FIG. 1 shows a first exemplary embodiment without argon production and

FIG. 2 shows a second exemplary embodiment with argon production.

The main air compressor, the precooling of the air and the air purification are not shown in FIG. 1. The purified total air 1 enters in the first mode of operation (standard operation/design case) at a pressure of 5.8 bar. A first part 2 is cooled to about dew point at this pressure in a main heat exchanger 3 and introduced via line 4 into the high-pressure column 5 of a distillation column system which, in addition, has a low-pressure column 6 and a main condenser 7. The two columns, at the top thereof have an operating pressure of 5.0 to 5.5 bar, or 1.3 to 1.4 bar, respectively. Alternatively, the pressures in the two columns can be elevated roughly proportionally to a higher level.

A second part 8 of the total air 1 is boosted to 58 bar in a pair of parallel-connected booster compressors 9, 10 having an aftercooler 11 and fed to the main heat exchanger 3 as “first substream” 13 and “second substream” 16. The first substream is conducted to the cold end of the main heat exchanger and in this case pseudo-liquefied. After expansion in a throttle valve 15 it is introduced into the high-pressure column 5 in predominantly liquid state. The second substream is withdrawn from the main heat exchanger 3 at an intermediate temperature via line 16, work-producingly expanded in an expansion turbine 1 to about high-pressure column pressure. After separating off a small liquid fraction in a separator (phase separator) 18, the second substream is fed to the high-pressure column together with the first part of the feed air via line 4. The turbine 17 is braked by an electric generator G.

The oxygen-enriched sump liquid 19 of the high-pressure column is cooled in a subcooling counterflow heat exchanger 20 end fed to an intermediate point via line 21 of the low-pressure column 6. Via lines 22 and 23, at least part of the air fed into the high-pressure column is removed again directly and, alter subcooling 20, is fed to the low-pressure column 6. Impure liquid nitrogen 24 is likewise subcooled (20) and then applied via line 25 as reflux to the top of the low-pressure column 6.

A first part 27 of the gaseous overhead nitrogen 26 of the high-pressure column 5 is completely or virtually completely liquefied in the main condenser 7. The liquid nitrogen 28 produced in this case is applied as a first part 29 as reflux to the top of the high-pressure column 5. A second part 30, 32 can be produced after subcooling 20 and flash gas separation in a separator (phase separator) 33 be produced as liquid product (LIN). A second part 39 of the gaseous overhead nitrogen 26 of the high-pressure column 5 is warmed in the main heat exchanger and obtained via line 40 as gaseous compressed nitrogen product (PGAN).

From the bottom of the low-pressure column (more precisely: from the vaporization space of the main condenser 7), liquid oxygen 34 is taken off. A first part thereof flows as “first oxygen stream” 35 to a pump 36 and there is pressurized to a pressure of 30 bar in the liquid state. The (subcritical in the example) oxygen stream 37 is conducted to the cold end of the main heat exchanger. In the main heat exchanger 3 it is vaporized and warmed to about ambient temperature. Via line 38, the first oxygen stream is finally obtained as gaseous compressed oxygen product (GOX IC).

A second part 44/45 of the liquid oxygen 34 is—optionally after subcooling 20—taken off via line 45 as “third oxygen stream” and obtained as liquid product. It is introduced, in particular, into a liquid oxygen tank (which is not shown) (LOX to tank).

A line 46 serves for feeding a “second oxygen stream” from the liquid oxygen tank into the bottom of the low-pressure column; however, in the first mode of operation it is out of operation.

Gaseous impure nitrogen 41 from the top of the low-pressure column 6 is warmed in the subcooling counterflow heat exchanger 20 and further in the main heat exchanger 3 and blown off to atmosphere via line 42 or used as regeneration gas in the air purification appliance which is not shown.

In the first mode of operation, the air turbine 17 is in operation, and flow does not pass through the bypass line 43. Likewise, liquid oxygen is taken off from, the distillation system via line 45. In addition, nitrogen can be produced as liquid product (LIN) and also gaseous nitrogen from the low-pressure column (which is not shown).

In a second mode of operation (electric power saving operation), the line 45 is closed; preferably, also, no liquid nitrogen (LIN) is produced. Conversely, via line 46, liquid oxygen is fed from outside the distillation column system into the low-pressure column. The product amount of gaseous compressed oxygen 38/GOX IC remains the same in this case. The total air amount 1 is decreased by about 32 mol % in comparison with the first mode of operation, and the second part 8/12 even by 65 mol %; preferably, one of the two booster compressors 9, 10 is out of operation, the other is run at a reduced output. The turbine 17 is stationary, the bypass 43 is open and a small stream flows therethrough which purges the corresponding passages of the main heat exchanger. The total air pressure is only 5.3 bar, the air pressure downstream of the booster compressors 9, 10 is only 53 bar. In this case, in the second mode of operation, the same amount of gaseous compressed oxygen product (GOX IC) is delivered at the same pressure as in the first mode of operation. These figures apply in the event that, in the first mode of operation, about 25 mol % of the total oxygen product is produced as liquid product and about 75 mol % as gaseous (internally compressed) compressed product at approximately 30 bar. Furthermore, in this case, roughly the same amount of liquid nitrogen is produced as of liquid oxygen. Here, two effects reinforce each other and thereby permit a particularly high decrease in the energy consumption at the main air compressor (total air amount) and in the booster compressors (first arid second substream): firstly, the total air amount is decreased, in that liquid oxygen is fed in from outside (and therefore no longer needs to be produced from the air amount fed in); secondly, the LOX and LIN products that are not produced further decrease the air and cold requirements. In the second numerical example shown below for a pure gas plant, in contrast, only the reductions in amounts are described which are solely due to feeding in the external LOX in the second mode of operation.

In the context of the invention, a pure gas plant (second mode of operation) can be made from the plant for generating liquid products (first mode of operation) and in this case, in times of high electric power prices, a lot of energy can be saved. This method remains efficient here, since none of the compressors in the bypass is operated arid the losses on throttling the turbine stream, because of the small (predominantly required for the purging of heat exchanger passages) amount and the low intake temperature (this temperature in the second mode of operation is substantially lower than in the first), are relatively small. An effective operating mode without liquid production is virtually made possible. Additional energy savings come from the reduced total air amount (correspondingly reduced drive energy at the main air compressor which is not shown). Because of unneeded cold output, in addition, drive energy can be saved in the booster compressors 9/10.

In the context of the invention, an existing liquid plant according to FIG. 1, but without line 46, can be appropriately retrofitted. For this purpose, only the installation of this line 46 is necessary, otherwise all components remain the same.

The invention can also be used correspondingly in methods without booster compression, in which the total air is compressed to markedly above high-pressure column pressure (HAP—high air pressure), independently thereof, the turbine 17, instead of the generator, can be braked by a booster compressor for turbine air. Also, an application of the invention to methods with what is termed a blast-intake turbine (the air from the main air compressor, after expansion, is not passed into the pressure column, but into the low-pressure column), or with more than one turbine, and also to those having a nitrogen circuit is possible.

FIG. 2 is differentiated from FIG. 1 only by an added argon production, which here is only shown schematically (argon box). This is connected in the usual manner to high-pressure column and low-pressure column.

In a first numerical example, the plant can be operated according to FIG. 2 as in FIG. 1. In this case, in the second mode of operation, an amount of liquid argon LAR is produced, which is proportional to the total air amount.

A second numerical example differs therefrom in that (also) in the first mode of operation, no liquid oxygen product is produced (and preferably also no liquid nitrogen product LIN). In this case also, the product amount of gaseous compressed oxygen 38/GOX IC in the second mode of operation is equal to that in the first mode of operation. The total air amount, in comparison with the first mode of operation, is reduced by 10 mol %, the second part 8/12 by 25 mol %. This can also be effected using a single booster compressor (instead of the two parallel-connected booster compressors shown in the drawings).

In departure from the presentation in the drawings, the turbine stream 16 can also be taken off at an intermediate take off of the two booster compressors 9, 10, that is to say at a lower pressure than the pressure than the throttle stream 33, which is taken off from the exit of the booster compressors 9, 10. In principle, the turbine 17 can also be braked using a booster compressor stage, which further booster compresses one or both of the streams 13 and 16. 

1. A method for producing gaseous compressed oxygen having variable power consumption by low temperature separation of air in a distillation column system that has a high-pressure column and a low-pressure column, in which feed air in the form of a total air stream is cooled in a main heat exchanger, at least a part, of the cooled feed air is introduced into the high-pressure column, a first oxygen stream from the low-pressure column is pressurized in the liquid state, the pressurized first oxygen stream is vaporized or pseudo-vaporized and warmed in the main heat exchanger, the warmed first oxygen stream is obtained as a gaseous compressed oxygen product, a first substream of the feed air, before entry thereof into the main heat exchanger, is brought to a first high pressure which is at least 4 bar higher than the operating pressure of the high-pressure column, the first substream is liquefied or pseudo-liquefied at the first high pressure in the main heat exchanger and subsequently introduced into the distillation column system, a second substream of the feed air is brought to a second high pressure that is at least 4 bar higher than the operating pressure of the high-pressure column, the second substream is cooled in the main heat exchanger only to an intermediate temperature, the second substream that is cooled to the intermediate temperature is work-producingly expanded and subsequently introduced into the distillation column system, wherein, in a first mode of operation a first total air amount is cooled in the main beat exchanger, a first turbine amount, as first substream, is fed to the work-producing expansion, and, wherein in a second mode of operation a second total air amount is cooled in the main beat exchanger, which second total air amount is less than the first total air amount of, a second turbine amount is fed as second substream to the work-producing expansion which second turbine amount is less than the first turbine amount characterised in that in the second mode of operation, a second oxygen stream is introduced from an external source outside the distillation column system into the low-pressure column in the liquid state and in the first mode of operation, the first and second substreams are together boosted in a pair of parallel-connected boosters.
 2. The method as claimed in claim 1, characterized in that at least, one of the following conditions is met: the second total air amount is at least 5 mol % lower than the first total air amount, a second turbine amount is at least 10 mol % lower than the first turbine amount.
 3. The method as claimed in claim 1, characterized in that in the first mode of operation, a third oxygen stream is taken off as liquid product from the low-pressure column in the scope of a first liquid oxygen amount and in the second mode of operation, the third oxygen stream is taken off as liquid product in the scope of a second liquid oxygen amount which is lower than the first liquid oxygen amount, wherein the second liquid oxygen amount is lower than the first liquid oxygen amount.
 4. The method as claimed in claim 1, characterized in that, in the second mode of operation, none of the process streams of the distillation column system is subjected to a cold compression.
 5. The method as claimed in claim 1, characterized in that the second turbine amount is zero.
 6. The method as claimed in claim 1, characterized in that the two boosters have a shared aftercooler or have an aftercooler each.
 7. The method as claimed in claim 1, characterized in that the total air stream consists of a first part and a second part wherein the second part consists of the first substream and the second substream, and in particular wherein the first part is fed without turbine expansion substantially in the gaseous state into the distillation column system.
 8. A device for producing gaseous compressed oxygen having variable energy consumption by low-temperature separation of air having a distillation column system that has a high-pressure column and a low-pressure column, having a main heat exchanger for cooling feed air in the form of a total air stream, having means for introducing at least a pan of the cooled feed air into the high-pressure column, having means for pressurizing a first oxygen stream from, the low-pressure column in the liquid state, having means for vaporizing or pseudo-vaporizing and warming in the main heat exchanger the pressurized first oxygen stream, having means for producing the warmed first oxygen stream as a gaseous compressed oxygen product, having means for bringing a first substream of the feed air, before entry thereof into the main heat exchanger, to a first high pressure which is at least 4 bar higher than the operating pressure of the high-pressure column, having means for liquefying or pseudo-liquefying the first substream liquefied or pseudo-liquefied at the first high pressure in the main heat exchanger, having means for introducing the (pseudo-)liquefied first substream into the distillation column system introduced, having means for bringing a second substream of the feed air to a second high pressure which is at least 4 bar higher than the operating pressure of the high-pressure column, having means for withdrawing the second substream in the main heat exchanger at an intermediate temperature, having means for the work-producing expansion of the second substream that is cooled to the intermediate temperature, having means for introducing the work-producingly expanded first substream into the distillation column system introduced characterized by means for introducing a second oxygen stream in the liquid state from an external source outside the distillation column system into the low-pressure column and by a control device, by which the following process parameters are set: in a first mode of operation a first total air amount which is cooled in the main heat exchanger, a first turbine amount which is fed as first substream to the work-producing expansion, that in a second mode of operation and in a second mode of operation a second total air amount, is cooled in the main heat exchanger which is less than the first total air amount, a second turbine amount is fed as first substream to the work-producing expansion, which second turbine amount is less than the first turbine amount an amount of the second oxygen stream which is fed to the low-pressure column in the liquid state, which amount is greater than the amount in the first mode of operation.
 9. A method for retrofitting a low-temperature air separation plant for producing gaseous compressed oxygen having variable power consumption by Low temperature separation of air in a distillation column system that has a high-pressure Column and a low pressure column in which feed air in the form of a total air stream is cooled in a main heat exchanger, at least a part of the cooled feed air is introduced into the high pressure column, a first oxygen stream from the low pressure column is pressurized in the liquid state, The pressurized first oxygen stream is vaporized or pseudo-vaporized and warmed in the main heat exchanger, the warmed first oxygen stream is obtained as a gaseous compressed oxygen product, a first substream of the feed air before entry thereof into the main heat exchanger, is brought to a first pressure which is at least 4 bar higher than the operating pressure of the high-pressure column, the first substream is liquefied or pseudo-liquefied at the first high pressure in the main heat exchanger and subsequently introduced into the distillation column system, a second substream of the feed air is brought to a second high pressure that is at least 4 bar higher than the operating pressure of the high pressure column, the second substream is cooled in the main heat exchanger only to an intermediate temperature, the second substream that is cooled to the intermediate temperature is work-producingly expanded and subsequently introduced into the distillation column system, wherein in a first mode of operation a first total air amount is cooled in the main heat exchanger, a first turbine amount as first substream is fed to the work-producing expansion, and, wherein, in a second mode of operation a second total air amount is cooled in the main heat exchanger, which second total air amount is less than the first total air amount of, a second turbine amount is fed as second substream to the work-producing expansion which second turbine amount is less than the first turbine amount; wherein in the second mode of operation, a second oxygen stream is introduced from an external source outside the distillation column system into the low-pressure column in the liquid state and in the first mode of operation, the first and second substreams are together boosted in a pair of parallel-connected boosters; the retrofitting method characterized in that means are added for introducing the second oxygen stream into the low-pressure column.
 10. The method as claimed in claim 9, characterized in that a further booster is connected in parallel to an existing booster.
 11. The method as claimed in claim 9, characterized in that, apart from the means for introducing the second oxygen stream into the low-pressure column and optionally apart from the further booster, no, or substantially no, changes to the device are made.
 12. The method as claimed in claim 2, wherein the second turbine amount is at least 30 mol % lower, than the first turbine amount.
 13. The method as claimed in claim 3, wherein the second liquid oxygen amount is at least 50 mol % lower than the first liquid oxygen amount.
 14. The method as claimed in claim 3, wherein the second liquid oxygen amount is 100 mol % lower than the first liquid oxygen amount.
 15. The method as claimed in claim 7, wherein the first part is fed without turbine expansion substantially in the gaseous state into the high pressure column. 